Tuesday, December 30, 2008
The Amazing Aurora
In Alaska in March, 2005, I took the above photo of an auroral display just outside Chena Hot Springs, Alaska. I was glad my wife was with me to behold the sight (her first time) since it later evolved to an exciting electronic scene with bursts of red and green interacting and changing positions.
How are these magnificent displays caused?
One can visualize the Earth as a giant spherical magnet, with magnetic
field lines extending from its north to south magnetic poles. These
magnetic field lines, have the property that any charged particles (+
protons, - electron or ions) that approach, will spiral along them.
The Earth itself, is "bathed" in the solar wind, a stream of high speed
charged particles that flows into space, originating from the Sun's
corona. (A hot, gaseous envelope that spews these particles out continuously – moreso when there is a violent explosion known as a Solar Flare)
Around the Earth the speed of these particles can reach 400- 500
km/second. (Because of its high temperature, over a million degrees, the
corona gas is *ionized* so must consist of charged particles, mainly (+)
protons, and (-) electrons).
During high solar activity (e.g. near sunspot maximums) a higher flux of
these charged particles inundates the solar wind, and the region around
The Earth's magnetic field traps these charged particles, and the highest
density is around the polar regions - which we refer to as the "auroral
ovals". In these regions, very high electric currents are set up, as the
charged particles start moving in unison about the magnetic field lines.
These currents can easily reach a few MILLION amperes.
As this discharge occurs, one or more outer electrons is stripped from the
atoms, for example from oxygen in the atmosphere - then RECOMBINES again -
to form new )e.g. oxygen) atoms.
With this RECOMBINATION - there is EMISSION of light, for a certain part
of the visible spectrum.
For example, in the case of recombination of oxygen atoms - their emitted
light is in the GREEN region of the spectrum. The aurora or northern
lights we see displays a kind of green curtain-like shimmering. The remarkable
red aurora is produced by emission at the 630 nm (nanometer) line of oxygen and at relatively high altitudes (e.g. 200-600 km) compared to green - which tends to form below 100 km and the oxygen line at 557 nm is excited.
Auroras can display as both diffuse and discrete. In the first case the shape is ill-defined and the aurora is believed to be formed from trapped particles originally in the magnetosphere which then propagate into the lower ionosphere via wave-particle interactions.
Thus, multiple colored auroras can be explained by emissions from
different atoms in the upper atmosphere, mainly in the region of the
magnetic poles. This is also why, of course, they are more often seen in
the vicinity of the N, S magnetic poles. (Though there have been reports
of N. lights being seen as far south as northern Florida, especially
during periods of esxceptional sunspot activity or LARGE SOLAR FLARES,
massive explosions on the Sun).
A great analogy has been given – by Syun Akasofu- comparing the aurora to images on a TV screen. In this case the (polar) upper atmosphere corresponds to the screen and the aurora to the image that would be projected on it, say for a TV. The electron beam in the TV (remember we are talking about the old-style cathode ray jobs!) corresponds the electron beam in the magnetosphere. In the conventional TV motions of the image are generated by the changing impact points of the electron beam on the screen. Similarly, with the aurora, its motions – such as moving sheets or curtains- are produced by moving impact points of the magnetospheric electron beams.
In gauging the power and intensity of auroras at different times, it is useful to remember that ultimately the aurora derives its power and potential from the Sun and specifically the charged particles of the solar wind. This is why the most spectacular displays are usually near sunspot maximum. Around those times the currents I noted earlier are “amped” up – no pun intended- to 10^6 A or more. To give an example, during a quiet Sun interval like we are in now the residual power for the magnetospheric generator is on the order of maybe a tenth of a megawatt. If we see a new cycle coming on and solar wind activated – we may get that power up to a million megawatts for a few hours.
If intense enough, such solar storms can herald the onset of enormous induction currents such as caused parts of the Ottawa grid to melt down in 1989.
But, as solar cycle 23 slowly ramps up, the aurora - of whatever color or shape- will be eagerly anticipated.